For the manufacture of integrated circuits it is necessary to image a number of masks with different circuits patterns upon a particular location of the semiconductor substrate.
This results in the exposure of a photosensitive layer of the substrate which serves after its development to cover the substrate at desired locations in chemical and physical process steps, such as etching and diffusion operations, carried out between imagings of different masks. A high degree of precision is required for the manufacture of the integrated circuitries. The permissible deviations of the successive imagings of the mask patterns lie for example below one micron. In order to obtain such a precision, the circuit patterns disposed on the mask are most often imaged on the substrate in reduced fashion, e.g. reduced by a factor of 10, via a projection objective. Especially with highly integrated circuits it has been found to be advantageous if each chip, i.e. each identical circuit, is exposed individually.
Pursuant to a known process, the semiconductor substrate is adjusted for this purpose with reference to an imaginary coordinate system fixedly tied to the projection-exposure equipment. With the aid of precise shifting devices (e.g. with the aid of a laser-interferometrically controlled X-Y table) the areas of the substrate corresponding to the individual circuits are pushed under the projection objective without any readjustment. The disadvantage of this indirect adjustment is evident since even at the first exposure the maladjustment of the corresponding substrate area relative to the coordinate system and the maladjustment of the mask relative to the coordinate system are added to each other. The deviations due to temperature fluctuations, for example, are not taken into consideration.
It has further become known to adjust the substrate relatively to the mask directly by way of the projection objective prior to the exposure of the individual areas corresponding to the respective chips. The movement of the substrate for the exposure or imaging of the mask pattern on the predetermined areas then occurs as described above. A drawback remains, however, in that deviations caused by disturbances during traverse of the predetermined areas are not taken into consideration.
Finally it has already been proposed to assign to the substrate individual adjustment marks for each imaging area, i.e. for each chip.
After each imaging of the circuit pattern and the subsequent shifting of the substrate to the next predetermined area, the substrate and the mask are adjusted by way of the projection objective. Though this procedure avoids the disadvantages inherent in the one referred to above, the overall processing time for a substrate is increased by the sum of the individual adjustment times. This causes an uneconomical reduction in throughput.